Control of Multilevel STATCOMs

Control strategies suitable for multilevel STATCOMs are reviewed in this chapter. First of all, the models are systematically deducted and especial emphasis is given to the dynamic and steady state analysis in order to facilitate the study of the control techniques. Then, these control schemes aimed to achieve the overall compensation objectives are classified in linear and nonlinear approaches. In fact, among the linear alternatives, the Proportional-Resonant (PR) and the Proportional-Integral (PI) controllers are presented as feasible ways to achieve zero steady state error in the stationary and the Synchronous Reference Frame (SRF), respectively. For nonlinear strategies, the input/output linearization control technique is applied. Hysteresis and predictive controllers are also discussed as alternatives to control the STATCOM in the stationary abc or α-β frame. The main features of all the aforementioned control techniques will be presented and because the balanced operation of the modules is not natural, dedicated control strategies in charge of balancing the DC side variables, DC voltages for Voltage Source Converters (VSC) and DC currents for Current Source Converters (CSC) based topologies, are shown as an important part of the control of the multilevel STATCOM. Indeed, the inclusion of these dedicated controllers to balance the operation of the different modules is mandatory for multilevel compensators if high performance overall waveforms are required. In order to illustrate the controllers performance operation, key current and voltage waveforms are shown for linear load compensation, using Cascade H-Bridge (CHB), Neutral Point Clamped (NPC) and Multilevel Current Source Converter (MCSC) based STATCOMs.